Page 166 - Design of Simple and Robust Process Plants

5.3 Combination of Process Functions 151
and 1989). This is an example of how complete conversion can be achieved by
removal of the product, and is equivalent to the reactive distillation technique. The
disadvantage is the circulation system that is needed for the solid adsorbent, and as
yet this technique has not yet be applied at industrial scale.

5.3.1.6 Reactive absorption
This has been in use for many years for the absorption of SO 3 in water for H 2 SO 4
production and NO x absorption in water for HNO 3 production. The removal pro-
cesses for CO 2 and H 2 S from gases with different reactants has also been practiced
for many years. It might be argued whether this process is really a combination of
functionality, as this also might be described as a G/L reactor. The removal of CO 2
from flue-gas through a membrane absorber (Feron and Janssen, 1995) also falls
into this category.
A combination of the objective functionalities of reactive absorption includes
membrane reactors, and these are currently under development (van de Graaf et al.,
1999). These reactors focus on equilibrium gas reactions (gases have higher permea-
tion fluxes), where reaction products are removed to achieve high conversions. In
essence, they might also be applied for the addition of a reactant to obtain high con-
versions in equilibrium reactions. Applications of this technique include:
. Hydrogen removal, as in steam reforming, water gas shift reactions, propane
dehydrogenation or ethyl benzene dehydrogenation by removal of hydrogen
between reaction stages.
. Oxygen addition, as in syngas production and methanol synthesis, as well as
the metathesis of propene from ethene and butene.
A summary of the integration of reaction and other process functions is shown in
Table 5.1.

Table 5.1. Combination of reaction function with other process functions, and some applications.

Reaction Applications(s)

Reactive distillation Esterification, Etherification (MTBE)
Reactive extrusion Co-polymerization
Reactive extraction Bromination of dialcohols
Reactor integrated exchangers (reverse flow) Catalytic combustion, Syngas production
Reactive adsorption Methanol synthesis
Reactive absorption HNO 3 ,H 2 SO 4 production, sourgas purification
Reaction / reaction Mixed ion-exchange, ethyl benzene dehydrogena-
tion with inter-stage hydrogen oxidation